Photonic integrated circuits (PIC) provide an integrated technology platform increasingly used to form complex optical circuits. PIC technology allows many optical devices, both active and passive, to be integrated on a single substrate. For example, PICs may comprise integrated lasers, integrated receivers, waveguides, detectors, semiconductor optical amplifiers (SOA), and other active and passive semiconductor optical devices. Monolithic integration of active and passive devices in PICs provides an effective integrated technology platform for use in optical communications.
A particularly versatile PIC platform technology is the integrated twin waveguide (TG) structure. Twin waveguides combine active and passive waveguides in a vertical directional coupler geometry using evanescent field coupling. The TG structure requires only a single epitaxial growth step to produce a structure on which active and passive devices are layered and fabricated. That is, TG provides a platform technology by which a variety of PICs, each with different layouts and components, can be fabricated from the same base wafer. Integrated components are defined by post-growth patterning, eliminating the need for epitaxial regrowth. Additionally, the active and passive components in a TG-based PIC can be separately optimized with post-growth processing steps used to determine the location and type of devices on the PIC.
The conventional TG structure, however, suffers from the disadvantage that waveguide coupling is strongly dependent on device length, due to interaction between optical modes. For PIC devices such as lasers, the interaction between optical modes results in an inability to control the lasing threshold current and coupling to passive waveguides as a consequence of the sensitivity to variations in the device structure itself. The sensitivity variations arise from the interaction between the even and the odd modes of propagation in the conventional TG structure. This interaction leads to constructive and destructive interference in the laser cavity, which affects the threshold current, modal gain, coupling efficiency and output coupling parameters of the device. The conventional TG structure suffers from unstable sensitivity in performance characteristics due to device length, even/odd mode interaction, and variations in the layered structure.
A modified TG structure, referred to as an asymmetric twin waveguide (ATG), disclosed in US Patent Application Ser. No. 09,337,785, filed on Jun. 22, 1999, entitled "Twin Waveguide Based Design for Photonic Integrated Circuits," the contents of which are hereby incorporated by reference in their entirety, addresses some of the performance problems of the conventional TG structure. The ATG structure significantly reduces modal interference by confining different modes of light to propagation in different waveguides. This is accomplished by designing each of the single mode waveguides that are comprised in the twin waveguide such that the mode of light that propagates in each of the two waveguides has a different effective index of refraction. The asymmetric waveguides may be laterally tapered to reduce coupling losses by resonant or adiabatic coupling of the optical energy between the first and second waveguide. The asymmetric waveguide design significantly reduces the interaction between optical modes and therefore represents a great improvement over traditional TG devices.
The ATG promises to be a versatile platform technology. Indeed, the inventors in an article entitled "Efficient Coupling in Integrated Twin-Waveguide Lasers Using Waveguide Tapers" published in volume 11, number 9, of the IEEE Photonic Technology Letters (September 1999), the contents of which are hereby incorporated by reference, suggest a design for integrating lasers using waveguides. Further, in an article entitled "Asymmetric Twin-Waveguide 1.55-.mu.m Wavelength Laser with a Distributed Bragg Reflector," published in volume 12, number 5, IEEE Photonic Tech. Letters (May 2000), the contents of which are hereby incorporated by reference, the inventors have disclosed designs for a laser device based upon the asymmetric twin waveguide design.
While these developments show promise for the ATG design, the need exists to develop the complex circuits often touted but, as of yet, not realized from PIC technology. Specifically, there is a need for improved PIC platforms that combine multiple optical devices, both active and passive, on a single substrate. More particularly, as described in an article by Nisa Hhan and Jim Rue entitled "Manufacturers Focus on the 40 Gbit/s Challenge," published in the June/July 2000 edition of FiberSystems International, the contents of which hereby incorporated, there is a need in the art for improved integrated photonic devices such as high speed detectors. Furthermore, there is a need to integrate photo-optical amplifiers with detectors so as to improve detector responsivity.